1. Field of the Invention
The invention relates to an image forming apparatus such as a copying machine wherein a charging member and a transfer member are movable individually using a single driving device in both directions of contacting with and separating from a photoreceptor, and more particularly to a driving apparatus and its control system for enabling such movement of the charging member and the transfer member.
2. Discussion of the Background
In an image forming apparatus wherein an electrophotographic method is employed such as a copying machine, a method of charging a photoreceptor with an electrostatic charge called a contact charging method has become popular in recent years. It is a method of charging a photoreceptor uniformly by bringing a charging member, such as a charging roller or a charging blade into contact with the photoreceptor. It has an advantage over a conventional charging method by corotron in that it prevents emission of ozone which is harmful to the human body, and also an advantage of being capable of charging using a relatively low voltage.
However, there exists a problem with this method in that materials contained in the charging member such as a plastic material which has a bad influence upon the photoreceptor are oozed out causing a deterioration of the photoreceptor and an ununiformity of the electrostatic charge. This problem has been observed also in the transfer member such as a transfer belt or a transfer roller which is brought into contact with the photoreceptor.
In order to avoid such a problem, there have been proposed various methods of separating the charging member and the transfer member from the photoreceptor when the apparatus is not in operation and bringing such members into contact with the photoreceptor when the apparatus is in the operation of forming an image. FIG. 20 and FIG. 21 are schematic drawings showing an example of such a mechanism of driving the charging member and the transfer member in an image forming apparatus. In the driving mechanism shown in FIG. 20 and FIG. 21, the first solenoid 53 and the second solenoid 54 are employed respectively as a driving source for moving a charging member 51 and a transfer member 52 in both directions of contacting with and separating from a photoreceptor 50. As shown in FIG. 20, the charging member 51 is mounted on a link 56 which is rotatable around a fulcrum 55 and is normally separated from the photoreceptor 50 by the pulling power of a pulling spring 57. To the base end of the link 56 is connected the first solenoid 53 and the link 56 is rotated clockwise by the action of the first solenoid 53 to bring, coupled with the pressuring power of a pressuring spring 58, the charging member 51 into contact with the photoreceptor 50. The transfer member 52 is mounted on a link 60 which is rotatable around a fulcrum 59. At the middle part of the link 60 is contacted a link 62 which is rotatable around a fulcrum 61. Further, the second solenoid 54 is connected to the base end of the link 62 and a pulling spring 63 is connected at the top end. When the second solenoid 54 is off, the link 62 rotates clockwise by the pulling spring 63, lowering the link 60 downwardly to separate the charging member 52 from the photoreceptor 50 as shown in FIG. 20. When the second solenoid 54 is on, the link 62 rotates counter-clockwise, thereby rotating the link 60 counter-clockwise by the pressuring power from the pressuring spring 64 to bring the transfer member 52 into contact with the photoreceptor 50 as shown in FIG. 21.
As described above, movement of the charging member 51 and the transfer member 52 to the directions of contacting with and separating from the photoreceptor 50 are controlled by individual driving sources in the mechanism shown in FIG. 20 and FIG. 21. Therefore, there exists in such mechanism problems such as the mechanism becoming complicated, the size of the apparatus becoming large and the cost of the parts becoming high. Further, the control of driving such mechanism becomes complicated due to the necessities of controlling individual driving sources.
FIG. 22 and FIG. 23 are schematic drawings showing another example of driving a charging member and a transfer member, which has been proposed to overcome the above problems. In the mechanism shown in these drawings, a solenoid 70 is connected to the base end of the link 56 on which the charging member 51 is mounted and to the base end of the link 62 which gives movement to the transfer member 52 to contact with and separate from the photoreceptor 50. When the solenoid 70 is off, the charging member 51 is separated from the photoreceptor 50 by the pulling power from the pulling spring 57, and the transfer member 52 is separated from the photoreceptor 50 resisting to the pressuring power from the pressuring spring 64. When the solenoid 70 is on, the link 56 rotates clockwise, resisting the pulling power of the pulling spring 57, to bring the charging member 51 into contact with the photoreceptor 50, coupled with the pressuring power from the pressuring spring 58, and at the same time, the link 60, support for the same by the link 62 having been released, rotates counter-clockwise to bring the transfer member 52 into contact with the photoreceptor 50, coupled with the pressuring power from the pressuring spring 64.
Now, in an image forming apparatus such as copying machines, the density of the images being produced are inspected periodically by a photosensor such as a photosensor 71 illustrated in FIG. 23 for the purpose of maintaining the image density at a certain level every time a prescribed number of the image forming operations has been made. Namely, a standard image, hereinafter referred to as a pattern image is generated automatically on the photoreceptor (in a way well known in the industry) after a prescribed number of the image forming operations have been performed, and the image density of such a pattern image, after its development, is inspected by the photosensor provided in the apparatus. Various parameters of the apparatus are adjusted in accordance with the detected image so that the apparatus continues to produce images of the same density. In a conventional apparatus, the photosensor is placed between a developing unit and a transfer member, and it is normally designed so that the inspection by the photosensor is carried out before the pattern image, after being developed, reaches the point where the pattern image comes in contact with the transfer member.
However, as image forming apparatuses become more versatile in functions, such a design as to place the photosensor on the downstream of the transfer member has been employed in some apparatus of recent development. In such apparatuses, there occurs a problem in that the pattern image is destroyed by the transfer member before being inspected by the photosensor, if the transfer member is in contact with the photoreceptor when the pattern image reaches the point of contact with the photoreceptor.
According to the driving mechanism shown in FIG. 22 and FIG. 23, the transfer member 52 is in contact with the photoreceptor 50 when the charging member 51 is in contact with the photoreceptor 50, and likewise, the transfer member 52 is separated from the photoreceptor 50 when the charging member 51 is separated from the photoreceptor 50, since the charging member and the transfer member are both driven integrally by the same driving source and only two motions are available. With such a mechanism, in the case the photosensor 71 cannot be placed between the developing unit and the transfer member and has to be placed on the downstream of the transfer member 52, it becomes impossible to inspect the image density periodically for the purpose of maintaining the image density at the same level, since the pattern image 71p is destroyed by the transfer member 52 as described above with such a driving mechanism.